1. Field of the Invention
The present invention relates to ceramic materials, and more particularly densified ceramic materials and a method of producing them.
Impregnation of low density ceramic bodies to complete densification is typically accomplished by immersing a ceramic part in a bath of a chemical solution, slurry, or melt, often under vacuum to assist the removal of trapped air. Multiple immersions are commonly necessary to achieve sufficient penetration and densification. With these conventional impregnation techniques, elimination of the final, few percent porosity is extremely difficult or impossible because of the sealing-off of passageways to the interior of the ceramic body during the impregnation process. Sufficient densification may also be difficult due to the high viscosities of some impregnants or large slurry particle sizes relative to the available openings in the ceramic body.
2. Description of the Prior Art
Supercritical fluids are dense gases and liquids at conditions above their respective thermodynamic critical points. They exhibit a number of unique properties which form the basis for their usefulness as processing tools. They exhibit high solvent power for many normally insoluble substances, pressure dependent solvent power, near ambient temperature processing capability, low viscosity and high diffusivity, and the absence of surface tension. For any particular supercritical fluid, at sufficiently high pressure the isobaric solubility of a material increases as a function of temperature. At lower pressures, the solubility of a dissolved material is inversely related to temperature. At a given temperature (above the critical temperature of the fluid) a decrease in pressure reduces the solubility of the dissolved material in the fluid.
Supercritical fluids have been used for the recovery of certain materials from foodstuffs and other starting materials. U.S. Pat. No. 3,806,619 (Zosel) for example discloses the use of supercritical carbon dioxide for recovery of caffeine. U.S. Pat. No. 4,104,409 (Vitzhum et al.) describes the removal of certain resins from hops using supercritical carbon dioxide and other compounds. U.S. Pat. No. 4,167,589 (Vitzhum et al.) shows the impregnation of dearomatized, decaffeinated tea using supercritical fluids such as carbon dioxide. U.S. Pat. No. 4,354,922 (Derbyshire et al.) shows a dense gas solvent, in a supercritical fluid state above its critical temperature and pressure, used to extract heavy hydrocarbon oil constituents. The reference teaches that the pressure is lowered (while maintaining the temperature above the critical temperature), or the temperature is raised, to precipitate out the dissolved hydrocarbon constituents. It has thus been demonstrated that supercritical fluids are applicable for extracting normally insoluble materials and removing them from a base material. Vitzhum et al. U.S. Pat. No. 4,167,589 also teaches that supercritical carbon dioxide can absorb certain aromatic consituents of tea, and upon subsequent dissociation can redeposit these aromatics in the tea. U.S. Pat. No. 4,241,112 (Kostandov et al.) discloses the successive deposition of an organometallic catalyst on the surface of a solid filler. The deposition of the second component of the catalyst is gas or liquid phase deposition, and a simultaneous polymerization of olefins on a first deposited catalyst component is carried out at temperatures which in some cases fall within the supercritical regime.
While supercritical fluid technology has thus been demonstrated to be useful particularly in the removal of certain components by dissolution relating to foodstuffs, and deposition of an organometallic catalyst or redeposition of aromatics, none of the references have applied this technology to the specific densification of porous ceramic structures. It has been discovered that the highly permeative nature of supercritical fluids allows for much easier infiltration (penetration) of smaller openings in ceramic material and can thus improve the densification of ceramic bodies.